1. Field of the Invention
The present invention relates to a nitride semiconductor light-emitting device and a method for producing the same.
2. Description of the Background Art
A III-V group compound semiconductor containing nitrogen (hereinafter, referred to as “nitride semiconductor”) has a bandgap energy corresponding to the energy of light having a wavelength in the infrared region to the ultraviolet region. Therefore, the nitride semiconductor is useful as a material for a light-emitting device that emits light having a wavelength in the infrared region to the ultraviolet region or a material for a light-receiving device that receives light having a wavelength in the infrared region to the ultraviolet region.
Further, binding between atoms constituting the nitride semiconductor is strong, dielectric breakdown voltage of the nitride semiconductor is high, and a saturation electron velocity of the nitride semiconductor is high. Due to these factors, a nitride semiconductor is useful as a material for an electronic device such as a high temperature-resistant and high-output high-frequency transistor.
Further, the nitride semiconductor will rarely harm the environment, and hence attracts attention as an easily-handled material.
In a nitride semiconductor light-emitting device using such a nitride semiconductor, a quantum well structure is generally employed as a light-emitting layer. When a voltage is applied, an electron and a hole are recombined in a well layer in the light-emitting layer, and as a result, light is generated. The light-emitting layer may have a single quantum well structure, or may have a multiple quantum well (MQW) structure where a well layer and a barrier layer are alternately stacked.
In the meantime, it is known that there is a defect in the form of a so-called V pit (V-shaped pit), V defect, or inverted hexagonal pyramid defect in a nitride semiconductor light-emitting device using a nitride semiconductor. For example, Japanese Patent Laying-Open No. 2005-277423 discloses a structure in which a “hexagonal pyramid cavity” is formed in a surface of a LED chip.
Since a V pit is a defect, it is generally supposed that characteristics of the LED will be improved by preventing generation of a V pit. On the other hand, in A. Hangleiter, F. Hitzel, C. Netzel, D. Fuhrmann, U. Rossow, G. Ade, and P. Hinze, “Suppression of Nonradiative Recombination by V-Shaped Pits in GaInN/GaN Quantum Wells Produces a Large Increase in the Light Emission Efficiency”, Physical Review Letters 95, 127402 (2005), operation of a V pit (V-Shaped Pit) in a MQW light-emitting layer is reported. According to this, when a V pit exists in a MQW light-emitting layer, a quantum well width in the slant surface of the V pit is narrow. Therefore, an electron or a hole injected into the quantum well is prevented from reaching a threading dislocation which is a crystal defect inside the V pit, and as a result, non-light-emitting recombination in the MQW light-emitting layer is suppressed.
In M. Shiojiri, C. C. Chuo, J. T. Hsu, J. R. Yang and H. Saijo, “Structure and formation mechanism of V defects in multiple InGaN/GaN quantum well layers”, JOURNAL OF APPLIED PHYSICS 99, 073505 (2006), it is reported that an apical angle of a V pit is ideally 56°.
As a technique of doping a light-emitting layer with a conductive impurity, Japanese Patent Laying-Open No. 2005-109425 discloses that an active layer (corresponding to the light-emitting layer in the present application) is formed by sequentially stacking an undoped GaN barrier layer and an n-type-impurity-doped InGaN quantum well layer. Also this publication discloses that a diffusion preventive film is provided at the interface where the undoped GaN barrier layer is in contact with the aforementioned InGaN quantum well layer, and that the diffusion preventive film contains an n-type impurity at a concentration lower than that in the InGaN quantum well layer.
Japanese Patent Laying-Open No. 2000-349337 discloses that the active layer contains an n-type impurity, and n-type impurity concentration in the active layer is higher in the n-layer side than in the p-layer side. Also this publication discloses that since n-type impurity concentration in the active layer is higher in the n-layer side than in the p-layer side, it is possible to supply a donor from the n-layer side to the active layer, and to obtain a nitride semiconductor device having high light emission output.